1. Field of the Invention
The object of the present invention is an integrated circuit for the confidential storage and processing of data, comprising a device against fraudulent use. It pertains, more especially but not exclusively, to the integrated circuits of the above type, called memory cards.
2. Description of the Prior Art
Memory cards are made with an electronic integrated circuit comprising essentially an electrically programmable non-volatile memory often comprising non-modifiable confidential data, input/output means used to couple the circuit with a data-transfer device and processing means interconnected between the input/output means and the memory. In certain memory card applications, access to certain zones of the non-volatile memory must imperatively be protected by an access key such as a confidential code associated with certain reading or writing rights in these specific zones. There are many devices used to dissuade fraudulent persons who might try to discover the secret code or the key that gives access to a memory by conducting successive tests. The best known of these devices is probably the one which permanently memorizes the wrong keys and blocks the operation as soon as the number of errors exceeds a pre-determined value. However, this device is inefficient if used by itself. For, when a memory cell is programmed at "1", there is a variation in the current and, by observing the variation of the write current in the memory, it is possible to determine, with precision, that the right key has been entered, provided that this current is correctly limited so that no errors can be recorded. To remove this disadvantage, the current method used is to program a datum in all cases thus causing current to be consumed. In this case, a fraudulent person cannot distinguish beteen correct attemts and incorrect ones by measuring the external current consumed by the component. The fraudulent person, thus cannot be informed of the results of his action. This operation, although efficient, has a serious disadvantage. It requires a relatively large memory space to record correct attempts which, in fact, are the most frequent ones. Now, in the case of a non-volatile memory of the EPROM or similar type, it will not be possible to re-use this space. Furthermore, it is not necessary to keep a record of correct attempts which are not subsequently used, unlike wrong attempts which must be used to block the system.
To remove this disadvantage, the specialist might think of programming only one and the same non-volatile memory cell if a datum presented is correct and of programing different cells only when the information presented is incorrect. This provides for a substantial increase in memory capacity. However, a memory cell which has already been programmed has a power consumption curve which is different than that of a virgin memory cell. For a virgin memory cell with a non-volatile memory has a current consumption peak of about 2% at the start of the programming, and this peak is virtually re-absorbed after 2 milliseconds. Now this consumption peak no longer exists when the cell is reprogrammed. It is therefore possible for a fraudulent person to detect the passage of a right key.
The present invention, therefore, is aimed at removing this disadvantage while at the same time using only one point to program correct attempts.